For my thread:
http://elementscommunity.org/forum/design-theory/the-new-card-theory-thread/I agree with the above, somewhat. I don't think it's strictly OP (because technically, it simply gets the bigger -2 upgrade bonus). Rather, Brimstone Eater should be 3|1 to compensate for it retaining a cost; because, while this only applies to restricted PvP, Fire does not deserve a free creature to combo with Immolation.
Dragons are... weird. To also include the many remembered attempts at cards with themes like "Heroes" and "Titans", dragons have that awkward place in which every element has notably different direct synergies with different sizes of creatures. I respectfully point the finger at Phase Dragon being the only non-vanilla dragon because of Parallel Universe. My thought would be to add some sort of passive to every dragon, since I do think Parallel Universe is fairly priced so long as it cannot combine with Phase Dragon, which the only solution to that is its Immateriality.
Honestly, Grey Nymph is by far the most overpowered. In just about any creature-cost theory (if not all), the stats alone make up for its playing cost. While in some light its ability is basically a double-poison, it's ability cost doesn't even compensate for that. A few of the nymphs have strong self-synergy, and I think we should consider that as their focal point, and then make tweaks to them or cards that they synergize with. Making Grey Nymph a glass cannon (2hp) so that (even though it won't be the main used strategy) it can kill itself with its own ability, summon a horde of Malignant Cells, and also we need a mass-sacrifice card for Death I think to synergize with this now one-card possibility.
Graboid aside, Shrieker is fine. Anubis does need a buff, for both grades. I'd reduce the non-upgraded ability cost to 1
(making it that for both), and the upgrade gets better stats, maybe like 6|5 (now it's conveniently in range of being Lightning'd) and maybe a slight cost decrease.
The simplest one to answer. Blue Crawler vs Horned Frog. In my thread, I explain Life getting a creature and creature generation mechanic bonus. That is, creatures as well as spells or permanents that generate or allow the generation of creatures get a -1 cost. For Blue Crawler, it receives no bonus because Water's is perceived to be that of duality; so cards with a non-Water ability cost get cost even less for having an off-cost ability.
Another "token" comparison, I think that Photon needs to generate a single
when played, but only when played. As for Skeleton, I do think it should have no cost, but just that. Light, from what I recall, might have a passive bonus for its creatures. (I think in the thread I initially noted that it was some sort of "ignorance" bonus).